Interpretive Summary: Drift from aerial application of crop protection and production materials is influenced by both controllable factors such as nozzle type, spray pressure and uncontrollable conditions such as wind speed and atmospheric stability. Applicators are responsible for considering atmospheric conditions and adjusting operational factors to reduce the potential for drift. Generally, greater atmospheric stability (less potential for vertical mixing) is associated with greater drift potential. Meteorological variables were monitored at two Texas locations and probability assessments of daily atmospheric temperature inversions and atmospheric stability were developed. Generally, the greatest potential for drift due to inversion conditions occurred during the late afternoon and at wind speeds less than 2 m/s. These results provide guidance to applicators about the daily cycle of atmospheric stability and the most appropriate application times to avoid potential drift.

Technical Abstract:
Drift from aerial application of crop protection and production materials is influenced by many factors. The applicator is responsible for considering these factors and adjusting application techniques, where applicable, to reduce the potential for drift as much as possible. In an effort to study the uncontrollable factors and provide guidance for agricultural applicators, this study monitored and documented atmospheric conditions at two locations. The measured meteorological data was used to assess how atmospheric stability varied as a function of time of day, location, and other meteorological conditions. Additionally, inversion periods were examined for strength, time of occurrence, and duration. Stable and very stable atmospheric conditions, which would tend to produce the most drift, primarily occurred between the hours of 6 p.m. and 6 a.m., with a few occurrences between 6 a.m. and 6 p.m. Between the hours of 6 a.m. and 6 p.m. unstable atmospheric conditions tended to dominate. Of the days monitored, almost half experienced inversion periods between the hours of 6 a.m. and 6:30 p.m., with more than half of these inversion periods being after 4 p.m. and having durations an order of magnitude greater than periods of inversions seen between 6 a.m. and 4 p.m. Generally, these late afternoon periods are of most concern as the probability of experiencing increasingly stable conditions or long inversion periods increases. Based on these results, agricultural applicators should take caution when spraying in the morning or evenings, especially when wind speeds are below 2 m/s.